1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium, a method of producing the medium, and a magnetic storage device, and particularly, to a perpendicular magnetic recording medium including a magnetic layer in which magnetic particles are isolated by a non-magnetic material.
2. Description of the Related Art
Recently and continuing, magnetic storage devices, for example, hard disk drives, are widely used in computers because they have low prices per bit, and store digital signals, thus enabling an increase of their capacities. Along with rapidly increasing demand on the magnetic storage devices, especially due to applications of the magnetic storage devices to digital audio/image related appliances, it is required to further increase the capacity of the magnetic storage devices to store the video signals.
In order to achieve both a high capacity and a low price, attempts can be made to increase the recording density of a magnetic storage medium in the magnetic storage device, thereby making it possible to reduce the number of the magnetic storage media in the magnetic storage device. Moreover, by increasing the recording density, it is possible to reduce the number of magnetic heads and other parts, thereby reducing the price of the magnetic storage device.
The recording density of the magnetic storage medium can be increased by improving the signal-to-noise ratio (S/N) through increasing the recording resolution and reducing noise. In the related art, effects have been made at miniaturization of magnetic particles constituting a recording layer of the magnetic storage medium and magnetic isolation of the magnetic particles in order to reduce noise.
In a perpendicular magnetic recording medium, an underlayer formed from a soft magnetic material is applied on a substrate, and on the underlayer a recording layer is stacked, forming the perpendicular magnetic recording medium.
The recording layer is usually formed from a CoCr-based alloy, and is applied on the substrate by sputtering the CoCr-based alloy onto the substrate while continuously heating the substrate. In the CoCr-based alloy recording layer, there appear Co-enriched CoCr-based alloy magnetic particles, and non-magnetic Cr forming boundaries around the magnetic particles, whereby, adjacent magnetic particles are isolated.
On the other hand, when reproducing data from the perpendicular magnetic recording medium, the soft magnetic underlayer forms a magnetic circuit for magnetic flux to flow into a magnetic head. If the soft magnetic material is a crystal, magnetic domains are formed in the soft magnetic material, and spike noises are generated.
To reduce the noise, usually the soft magnetic underlayer is formed from materials in which it is difficult for magnetic domains to be formed, for example, amorphous materials or micro-granular crystals. Further, in order to avoid crystallization of the soft magnetic underlayer, the heating temperature is limited when forming the recording layer.
Therefore, in order to achieve isolation of the magnetic particles, it has been studied to use a recording layer which does not require high temperature heating. For example, in the recording layer, CoCr-based alloy magnetic particles are isolated by SiO2 non-magnetic parent phases. Furthermore, it is proposed that a Ru film be formed under the recording layer (below, referred to as an underlayer) so that the magnetic particles essentially grow at equal intervals. For example, Japanese Laid-Open Patent Application No. 2003-217107 and Japanese Laid-Open Patent Application No. 2003-346334 disclose inventions related to this technique.
However, if merely forming the Ru layer under the recording layer, crystals of the magnetic particles grow on the surface of the granular crystals of the Ru film, and depending on the sizes and arrangement of the granular crystals, the magnetic particles may combine with each other; as a result, sufficient isolation between the magnetic particles cannot be achieved, the distribution of diameters of the magnetic particles becomes more spread, and consequently, noise generated in the medium increases.
On the other hand, if adjacent magnetic particles are formed at regular intervals, it is necessary to form a seed layer below the Ru film to control growth of the granular crystals of the Ru film. In this case, a stacked structure of a plurality of seed layers is required, and this makes the seed layer thick. As a result, the distance between the soft magnetic underlayer and the recording layer is large, and this increases the magnetic field of the magnetic head required for recording. Further, because the distribution of the magnetic field of the magnetic head becomes more spread, data on neighboring tracks may be erased accidentally.